The first 5 points a Retribution Paladin generally spends are in this talent, all of our outgoing attacks are instant casts; Judgments, Exorcism, Divine Storm, Crusader Strike, Consecration, Hammer of Justice, Hammer of Wrath, Repentance, so this is of huge benefit for long running fights.
This talent is also useful to Holy Paladins, as it will reduce the mana cost of Holy Shock, instant Flash of Lights, Seals, Blessings and Judgements. It will also lower the cost of Beacon of Light which can be highly mana inefficient when kept up for long fights.
Protection Paladins will generally skip this talent in favor of Deflection, given that their mana levels can be well managed through Spiritual Attunement, Divine Plea, Judgement of Wisdom and Blessing of Sanctuary. Still, it can be a lifesaver when you are low on mana and need to cast a spell to prevent a wipe, as a paladin tank's mana pool is drained very fast when not under any special mana-regeneration effects.